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Summary

The sort() method sorts the elements of an array in place and returns the array. The sort is not necessarily stable. The default sort order is lexicographic (not numeric).

Syntax

arr.sort([compareFunction])

Parameters

compareFunction

Specifies a function that defines the sort order. If omitted, the array is sorted lexicographically (in dictionary order) according to the string conversion of each element.

Description

If compareFunction is not supplied, elements are sorted by converting them to strings and comparing strings in lexicographic ("dictionary" or "telephone book," not numerical) order. For example, "80" comes before "9" in lexicographic order, but in a numeric sort 9 comes before 80.

If compareFunction is supplied, the array elements are sorted according to the return value of the compare function. If a and b are two elements being compared, then:

If compareFunction(a, b) is less than 0, sort a to a lower index than b, i.e. a comes first.

If compareFunction(a, b) returns 0, leave a and b unchanged with respect to each other, but sorted with respect to all different elements. Note: the ECMAscript standard does not guarantee this behaviour, and thus not all browsers (e.g. Mozilla versions dating back to at least 2003) respect this.

If compareFunction(a, b) is greater than 0, sort b to a lower index than a.

compareFunction(a, b) must always return the same value when given a specific pair of elements a and b as its two arguments. If inconsistent results are returned then the sort order is undefined

So, the compare function has the following form:

function compare(a, b) {
if (a is less than b by some ordering criterion)
return -1;
if (a is greater than b by the ordering criterion)
return 1;
// a must be equal to b
return 0;
}

To compare numbers instead of strings, the compare function can simply subtract b from a:

function compareNumbers(a, b) {
return a - b;
}

The sort method can be conveniently used with {{jsxref("Operators/function", "function expressions")}} (and closures):

Sorting non-ASCII characters

For sorting strings with non-ASCII characters, i.e. strings with accented characters (e, é, è, a, ä, etc.), strings from languages other than English: use {{jsxref("String.localeCompare")}}. This function can compare those characters so they appear in the right order.

Sorting maps

The compareFunction can be invoked multiple times per element within the array. Depending on the compareFunction's nature, this may yield a high overhead. The more work a compareFunction does and the more elements there are to sort, the wiser it may be to consider using a map for sorting. The idea is to walk the array once to extract the actual values used for sorting into a temporary array, sort the temporary array and then walk the temporary array to bring the original array into the right order.